This work was supported by a Hong Kong Research Grant Council Grant (HKU 7656/07M), Hong Kong RGC Collaborative Research Grants (HKU5/CRF/08 and HKU7/CRG09) and the "Hundred Talents Program'' at Sun Yat-sen University (85000-3171311). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.